Neoplasia, also known as cancer, is the second most common cause of death in the United States. While the survival rates for individuals with cancer have increased considerably in the last few decades, survival of the disease is far from assured. Cancer is a catch-all term for over 100 different diseases, each of which are each fundamentally characterized by the unchecked proliferation of cells. Individual cancer cells are also able to break off from the main tumor, or metastasize, creating additional tumors in other regions of the body.
Due to the mortality rate and incidence of neoplasia in the general population, research into potential cures has been high on the national agenda for decades. This research has led to the development a number of treatments, both systemic and regional (local). Regional treatments include radiation therapy, some types of chemotherapy and surgery. Chemotherapy has most often been used in systemic treatment. Each of these treatment regimes has significant disadvantages and limitations. Chemotherapy and radiation treatments will be discussed below.
Chemotherapy
Chemotherapy refers to the use of chemical compounds or drugs in the treatment of disease, though the term chemotherapy is most often associated with the treatment of cancer. Cancer chemotherapeutic agents are also commonly referred to as antineoplastic agents. There are a number of classes of chemotherapeutic compounds, encompassing nearly 100 individual drugs, as well as numerous drug combination therapies, methods of delivery and schedules of treatment. Each of these chemotherapeutic agents may be classified according to several criteria, such as class of compound and disease state treated. Certain agents have been developed to take advantage of the rapid division of cancer cells and target specific phases in the cell cycle, providing another method of classification. Agents can also be grouped according to the type and severity of their side effects or method of delivery. However, the most common classification of chemotherapeutic agents is by class of compound, which broadly encompasses the mechanism of action of these compounds.
Depending on the reference source consulted, there are slight differences in the classification of antineoplastics. The classes of compounds are described in the Physician's Desk Reference as follows: alkaloids; alkylating agents; anti-tumor antibiotics; antimetabolites; hormones and hormone analogs; immunomodulators; photosensitizing agents; and miscellaneous other agents. Examples of these antineoplastics are listed in Table 1.
The alkaloid class of compounds are also referred to as mitotic inhibitors, as they are cell cycle phase specific and serve to inhibit mitosis or inhibit the enzymes required for mitosis. They are derived generally from plant alkaloids and other natural products and work during the M-phase of the cell cycle. This class of compounds is often used to treat neoplasias such as acute lymphoblastic leukemia, Hodgkin's and non-Hodgkin's lymphoma; neuroblastomas and cancers of the lung, breast and testes.
Alkylating agents make up a large class of chemotherapeutic agents, including of the following sub-classes, which each represent a number of individual drugs: alkyl sulfonates; aziridines; ethylenimines and methylmelamines; nitrogen mustards; nitrosoureas; and others. Alkylating agents attack neoplastic cells by directly alkylating the DNA of cells and therefore causing the DNA to be replication incompetent. This class of compounds is commonly used to treat a variety of diseases, including chronic leukemias, non-Hodgkin's lymphoma, Hodgkin's lymphoma, multiple myeloma and certain lung, breast and ovarian cancers.
Nitrosoureas are often categorized as alkylating agents, and have a similar mechanism of action, but instead of directly alkylating DNA, they inhibit DNA repair enzymes causing replication failure. These compounds have the advantage of being able to cross the blood-brain barrier and therefore can be used to treat brain tumors.
Antitumor antibiotics have antimicrobial and cytotoxic activity and also interfere with DNA by chemically inhibiting enzymes and mitosis or by altering cell membranes. They are not cell cycle phase specific and are widely used to treat a variety of cancers.
The antimetabolite class of antineoplastics interfere with the growth of DNA and RNA and are specific to the S-phase of the cell-cycle. They can be broken down further by type of compound, which include folic acid analogs, purine analogs, and pyrimidine analogs. They are often employed in the treatment of chronic leukemia, breast, ovary, and gastrointestinal tumors.
There are two classes of hormones or hormone analogs used as antineoplastic agents, the corticosteroid hormones and sex hormones. While some corticosteroid hormones can both kill cancer cells and slow the growth of tumors, and are used in the treatment of lymphoma, leukemias, etc., sex hormones function primarily to slow the growth of breast, prostate and endometrial cancers. There are numerous subclasses of hormones and hormone analogs, including, androgens, antiadrenals, antiandrogens, antiestrogens, aromatase inhibitors, estrogens, leutenizing hormone releasing hormone (LHRH) analogs and progestins.
An additional smaller class of antineoplastics is classified as immunotherapy. These are agents which are intended to stimulate the immune system to more effectively attack the neoplastic cells. This therapy is often used in combination with other therapies.
There are also a number of compounds, such as campothectins, which are generally listed as ‘other’ antineoplastic agents and can be used to treat a variety of neoplasias.
While there is a plethora of antineoplastic agents, the efficacy of these compounds is often outweighed by the severity of the side effects produced by the agent. This comparison is often referred to as the therapeutic index, which describes the balance between the required dose to accomplish the destruction of the cancer cells compared to the dose at which the substance is unacceptably toxic to the individual. The drawback to most antineoplastic agents is the relatively small range of the therapeutic index, (i.e, the narrow dosage range in which cancer cells are destroyed without unacceptable toxicity to the individual). This characteristic limits the frequency and dosage where an agent is useful, and often the side effects become intolerable before the cancer can be fully eradicated.
The severe side effects experienced with the majority of cancer chemotherapeutics are a result of the non-specific nature of these drugs, which do not distinguish between healthy and cancerous cells, and instead destroy both. The cell cycle specific drugs attempt to lessen these effects, targeting phases of the cell cycle involved in cell replication and division. These drugs do not, however, distinguish between cancerous cells and healthy cells which are undergoing normal cell division. The cells most at risk from these types of chemotherapy are those which undergo cell division often, including blood cells, hair follicle cells, and cells of the reproductive and digestive tracts.
The most common side effects of antineoplastic agents are nausea and vomiting. A large proportion of individuals also suffer from myelosuppression, or suppression of the bone marrow, which produces red blood cells, white blood cells and platelets. These and other side effects are also exacerbated by the suppression of the immune system concomitant with the destruction and lack of production of white blood cells, and associated risk of opportunistic infection.
Other side effects common to a wide range of antineoplastic agents include: hair loss (alopecia); appetite loss; weight loss; taste changes; stomatitis and esophagitis (inflammation and sores); constipation; diarrhea; fatigue; heart damage; nervous system changes; lung damage; reproductive tissue damage; liver damage; kidney and urinary system damage.
The wide range of the side effects associated with most antineoplastic agents and their severity in individuals who are already debilitated with disease and possibly immune compromised has led researches to search for mechanisms by which they can alleviate some of the side effects while maintaining the efficacy of the treatment. Several approaches to this problem have been taken. They include combination chemotherapy, where multiple antineoplastics are administered together; adjuvant therapies, where additional agents are prescribed along with the antineoplastic agent to fight the side effects of the antineoplastic; alternative delivery vehicles for the administration of chemotherapeutics, such as the encapsulation of antineoplastic agents in liposomes; and combined modality treatments, where chemotherapy is combined with radiation and/or surgery.
One difficulty with respect to combination chemotherapy is that many antineoplastic agents have similar side effects, so while their toxicity profiles are different, the individual will still suffer greatly and may not be able to finish the recommended course of treatment.
Another aspect of combination chemotherapy is the addition of hormones to the combination of drugs administered. While the hormone or hormonal analog treatment is generally not cytotoxic, hormonal manipulation helps to prevent or slow cell division and therefore slows the growth of the tumor. This type of therapy is often used for hormone dependent tumors of, for instance, the prostate, breast or ovaries. One well known example is the treatment of breast cancer with tamoxifen.
An additional method of combating the side effects associated with antineoplastics and, more importantly, extending the therapeutic dosage of these agents is adjuvant therapy, where additional agents are co-administered to the individual in order to ameliorate the side effects or toxicity of the antineoplastic agent. Examples of such adjuvant therapy includes the administration of chemoprotective agents, such as the uroprotective agent mesna, the antimetastatic agent batimastat, the folic acid replenisher folinic acid. Additional therapies include the administration of granulocyte colony stimulating factors, granulocyte-macrophage colony stimulating factor and even the transplantation of hematopoietic stem cells. These last three therapies aim to treat lessen the chance of opportunistic infection due to myelosuppression concomitant with many chemotherapy regimens. However, despite the recent advances in antineoplastic and adjuvant therapy there are still numerous cancers, for example ovarian cancer, that are resistant to current treatments, and leave the individual at risk for potentially serious infection.
Radiation Therapy
Along with chemotherapy and surgery, radiation is one of the most commonly used treatment modalities, used in approximately 60% of treatment regimens. Radiation, in any of several forms, is often used as the primary therapy for basal cell carcinomas of the skin, head and neck, prostate cancers, bladder cancers, and others. Often combined with chemotherapy and/or surgery, radiation therapy encompasses both local and total body administration as well as a number of new advances, including radioimmunotherapy.
The cytotoxic effect of radiation on neoplastic cells arises from the ability of radiation to cause a break in one or both strands of the DNA molecule inside the cells. Cells in all phases of the cell cycle are susceptible to this effect. However, the DNA damage is more likely to be lethal in cancerous cells because they are less capable of repairing DNA damage. Healthy cells, with functioning cell cycle check proteins and repair enzymes, are far more likely to be able to repair the radiation damage and function normally after treatment.
Tumors and tissues themselves are also characterized by a range of susceptibilities to radiation therapy. Lymphoma and leukemias are very sensitive to radiation therapy, while renal cancer and gland tumors are fairly insensitive to radiation. A tumor that is considered radiosensitive is one which can be eradicated by a dose(s) of radiation that is also well tolerated by the surrounding tissues. Unsurprisingly, different tissue types within the body tolerate radiation at different doses. Tissues that undergo frequent cell division are most effected by treatment, similar to their sensitivity to certain cell cycle specific chemotherapy agents.
The radiosensitivity of tumors is also effected by hypoxia, or a lack of oxygen in the interiors of larger tumors. Hypoxic tumors can be 2-3 times less responsive to radiation treatment. Certain agents used in conjunction with radiation treatment, such as some of the radiosensitizing agents, work by increasing the singlet oxygen species in the vicinity of the tumor and therefore increasing its radiosensitivity. Other compounds used in conjunction with radiation therapy include radioprotectants which are designed to protect surrounding tissue from some of the effects of radiation therapy. Sources of radiation include: Americium, chromic phosphate, radioactive, Cobalt, 131I-ethiodized oil, Gold (radioactive, colloidal) iobenguane, Radium, Radon, sodium iodide (radioactive), sodium phosphate (radioactive).
Radiation therapy itself can be classified according to two primary types, internal and external radiation therapy. External therapy involves the administration of radiation via a machine capable of producing high-energy external beam radiation. This therapy can include either total body irradiation, or can be localized to the region of the tumor. With external radiation treatments, the bodily secretions of the individual are not radioactive after treatment. The radiation itself can be either electromagnetic (X-ray or gamma radiation) or particulate (α or β particles). The treatment requirements will differ depending upon the characteristics of the tumor. External radiation is often used pre- or post-operatively; either to shrink the tumor before surgery, or to mop up remaining cancer cells after surgery.
Internal radiation therapy, also termed brachytherapy, involves implantation of a radioactive isotope as the source of the radiation. There a variety of methods of delivery, including permanent, temporary, sealed, unsealed, intracavity or interstitial implants. The choice of implant is determined by a variety of factors, including the location and extent of the tumor.
A third, but still experimental, type of radiation therapy is often termed radioimmunotherapy. This involves the attachment of radioisotopes to monoclonal antibodies specific for the tumor cells. Upon administration the antibodies specifically seek out and destroy the cancer cells.
The side effects of radiation are similar to those of chemotherapy and arise for the same reason, the damage of healthy tissue. Radiation is usually more localized than chemotherapy, but treatment is still accompanied by damage to previously healthy tissue. Many of the side effects are unpleasant, and radiation also shares with chemotherapy the disadvantage of being mutagenic, carcinogenic and teratogenic in its own right. While normal cells usually begin to recover from treatment within two hours of treatment, mutations may be induced in the genes of the healthy cells. These risks are elevated in certain tissues, such as those in the reproductive system. It has also been found that people tolerate radiation differently. Doses that may not lead to new cancers in one individual may in fact spawn additional cancers in another individual. This could be due to pre-existing mutations in cell cycle check proteins or repair enzymes, but current practice would not be able to predict at what dose a particular individual is at risk. Common side effects of radiation include: bladder irritation; fatigue; diarrhea; low blood counts; mouth irritation; taste alteration; loss of appetite; alopecia; skin irritation; change in pulmonary function; enteritis; sleep disorders; and others.
Adenovirus Vectors
Until relatively recently, the virtually exclusive focus in development of adenoviral vectors for gene therapy has been use of adenovirus merely as a vehicle for introducing the gene of interest, not as an effector in itself. Replication of adenovirus had previously been viewed as an undesirable result, largely due to the host immune response. More recently, however, the use of adenovirus vectors as effectors has been described. International Patent Application Nos. PCT/US98/04084, PCT/US98/04080; PCT/US98/04133, PCT/US98/04132, PCT/US98/16312, PCT/US95/00845, PCT/US96/10838, PCT/EP98/07380, U.S. Pat. No. 5,998,205 and U.S. Pat. No. 5,698,443. The use of IRES in vectors have been described. See, for example, International Patent Application No. PCT/US98/03699 and International Patent Application No. PCT/EP98/07380. Adenovirus E1A and E1B genes are disclosed in Rao et al. (1992, Proc. Natl. Acad. Sci. USA vol. 89: 7742-7746).
Publications describing various aspects of adenovirus biology and/or techniques relating to adenovirus include the following. PCT/US95/14461; Graham and Van de Eb (1973) Virology 52:456-467; Takiff et al. (1981) Lancet ii:832-834; Berkner and Sharp (1983) Nucleic Acid Research 6003-6020; Graham (1984) EMBO J 3:2917-2922; Bett et al. (1993) J. Virology 67:5911-5921; and Bett et al. (1994) Proc. Natl. Acad. Sci. USA 91:8802-8806 describe adenoviruses that have been genetically modified to produce replication-defective gene transfer vehicles. In these vehicles, the early adenovirus gene products E1A and E1B are deleted and provided in trans by the packaging cell line 293 developed by Frank Graham (Graham et al. (1987) J. Gen. Birol. 36:59-72 and Graham (1977) J. Genetic Virology 68:937-940). The gene to be transduced is commonly inserted into adenovirus in the deleted E1A and E1B region of the virus genome Bett et al. (1994), supra. Adenovirus vectors as vehicles for efficient transduction of genes have been described by Stratford-Perricaudet (1990) Human Gene Therapy 1:2-256; Rosenfeld (1991) Science 252:431-434; Wang et al. (1991) Adv. Exp. Med. Biol. 309:61-66; Jaffe et al. (1992) Nat Gent. 1:372-378; Quantin et al. (1992) Proc Natl. Acad. Sci. USA 89:2581-2584; Rosenfeld et al. (1992) Cell 68:143-155; Stratford-Perricaudet et al. (1992) J. Clin. Invest. 90:626-630; Le Gal La Salle et al. (1993) Science 259:988-990; Mastrangeli et al. (1993) J. Clin. Invest. 91:225-234; Ragot et al. (1993) Nature 361:647-650; Hayaski et al. (1994) J. Biol. Chem. 269:23872-23875.
There are several other experimental cancer therapies which utilize various aspects of adenovirus or adenovirus vectors. See, U.S. Pat. No. 5,776,743; U.S. Pat. No. 5,846,945; U.S. Pat. No. 5,801,029; PCT/US99/08592; U.S. Pat. No. 5,747,469; PCT/US98/03514; and PCT/US97/22036.
Of particular interest is the development of more specific, targeted forms of cancer therapy, especially in cancers that are difficult to treat successfully, such as prostate, bladder or ovarian cancer. In contrast to conventional cancer therapies, which result in relatively non-specific and often serious toxicity, more specific treatment modalities attempt to inhibit or kill malignant cells selectively while leaving healthy cells intact. There is, therefore a serious need for developing specific, less toxic cancer therapies.
All references cited herein are hereby incorporated by reference in their entirety.